1. Field of the Invention
The present invention relates to an optical signal transmission system, an optical signal transmitter, and an optical signal receiver, as well as a method of transmitting optical signal and a method of receiving optical signal. The present invention relates more particularly to an optical signal transmission system suitable for a wavelength-multiplexed optical network taking place a wavelength-multiplexing without depending upon a signal transmission rate and a transmission signal format.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
Typical one of the existent general communication networks is a synchronous digital hierarchy network, wherein a main signal is transmitted accompanying with an overhead which is to be used for maintenance and operation of the communication network. For example, J0 bite in the overhead is used as a trail identifier in order to monitor a correspondence between the signal transmission side and the signal receiving side, for the purpose of verifying a correct correspondence or an incorrect correspondence between the signal transmission side and the signal receiving side.
A switch such as a cross-connect switch for switching a signal transmission path may be provided in the synchronous digital hierarchy network. It is undesirable that any failure or any malfunction of the switch may cause a deficiency in correspondence relationship between the signal transmission side and the signal receiving side, This deficiency raises such a trouble that the signal may no longer be transmitted to an intended or desired destination in the receiving side. In order to avoid this trouble, it is effective to monitor undesired or unintended establishment of any incorrect connection between the transmitter and the receiver, wherein the monitoring process is made by use of the above-described trail identifier. If any incorrect connection or any signal disconnection is established and is detected, then a protection or a restoration is made to restore or recover the incorrectness, in order to keep a reliability of the network.
Not only the above-described synchronous digital hierarchy network as a time-multiplexing network but also the wavelength-multiplexed optical network need to monitor appearance of any incorrect connection or route between the signal transmitter and the signal receiver. The wavelength-multiplexed optical network is, however, advantageous in view of allowing the wavelength-multiplexing process to be independent from the transmission signal rate and the transmission signal format. The wavelength-multiplexed optical network does not avail a method for monitoring the correspondence between the signal transmitter and the signal receiver, provided that this method is similar to the method of using the overhead in the synchronous digital hierarchy network which accomplishes the signal-multiplexing in accordance with the fixed or predetermined frame format. The reason for the above unavailability or inapplicability of the method of using the overhead to the wavelength-multiplexed optical network would be that the wavelength-multiplexed optical network accepts any optional formats of the main optical signals, and it is impossible to extract the overhead-bit from the main optical signals.
Consequently, it is necessary for the wavelength-multiplexed optical network to monitor the correspondence between the signal transmitter and the signal receiver without, however, depending upon the format of the main optical signal.
Japanese laid-open patent publications Nos. 9-215072 and 11-55699 disclose conventional methods of monitoring the correspondence between the signal transmitter and the signal receiver in the wavelength-multiplexed optical network, wherein pilot tone signals assigned with frequencies specific to wavelengths of the main optical signals are superimposed as identifiers to the main optical signals so as to monitor a set state of an optical path and an optical signal level. The superimposition of the pilot tone signal over the main optical signal causes not only a deterioration in carrier-to-noise ratio of the pilot tone signal but also another deterioration in quality of the main optical signal. Simple consideration of only the presence or the absence of the pilot tone signal may provide undesired limitations to a maximum value of a transmittable distance of the main optical signal and another maximum value of a transmission rate of the main optical signal. Further, a laser diode is sub-carrier-modulated with the pilot tone signal for emitting a main optical signal superimposed with the pilot tone signal. This makes it necessary for replace the pilot tone signal as an identifier to take place a photoelectric conversion of the main optical signal. The photoelectric conversion of the main optical signal is essential for replacing the pilot tone signal as an identifier.
In the above circumstances, the development of novel optical signal transmission system, optical signal transmitter and method of transmitting optical signal, which are free from the above problems is desirable.